Because of environmental concerns, new energy sources, that are environmentally friendly and having good efficiency, have been developed. Fuel cell devices are promising future energy conversion devices by means of which fuel, for example bio gas, is directly transformed to electricity via a chemical reaction in an environmentally friendly process.
FIG. 1 illustrates a single fuel cell structure in accordance with a known device. Fuel cell, as presented in FIG. 1, includes an anode side 100 and a cathode side 102 and an electrolyte material 104 between them. In solid oxide fuel cells (SOFCs), oxygen is fed to the cathode side 102 and it is reduced to a negative oxygen ion by receiving electrons from the cathode. The negative oxygen ion goes through the electrolyte material 104 to the anode side 100 where it reacts with the used fuel producing water and carbon dioxide (CO2). Between the anode 100 and the cathode 102 is an external electric circuit 111 comprising a load 110 for the fuel cell.
FIG. 2 illustrates an example of a SOFC device in accordance with a known implementation. As shown in FIG. 2, an SOFC device is used as a high temperature fuel cell device. SOFC devices can utilize for example natural gas, bio gas, methanol, or other compounds containing hydrocarbon mixtures as fuel. The SOFC device system in FIG. 2 can include multiple fuel cells in one or more stack formation 103 (SOFC stack(s)). A larger SOFC device system includes many fuel cells in several stacks 103, where each fuel cell has anode 100 and cathode 102 structures. Part of the used fuel may be recirculated in feedback arrangement 109. SOFC device in FIG. 2 also includes a fuel heat exchanger 105 and a reformer 107. Heat exchangers are used for controlling thermal conditions in the fuel cell process and there can be more than one of them in different locations of a SOFC device. The extra thermal energy in circulating gas is recovered in one or more heat exchangers 105 to be utilized in the SOFC device or externally. Reformer 107 is a device that converts the fuel such as for example natural gas to a composition suitable for fuel cells, for example to a composition containing all or at least some of the following: hydrogen, methane, carbon dioxide, carbon monoxide, inert gases and water. Anyway in each SOFC device it is though not necessary to have a reformer.
By using measurement means 115 such as fuel flow meter, current meter and temperature meter measurements for the operation of the SOFC device are carried out. Only part of the gas used at the anodes 100 is recirculated in the feedback arrangement 109 and the other part of the gas is exhausted 114 from the anodes 100.
A solid oxide fuel cell (SOFC) device is an electrochemical conversion device that produces electricity directly from oxidizing a fuel. Advantages of SOFC device include high efficiencies, long term stability, low emissions, fuel versatility and cost. The main disadvantage is the high operating temperature which results in long start up times and both mechanical and chemical compatibility issues.
Fuel cell systems have the potential to significantly exceed the electrical and CHP (Combined production of Heat and Power) efficiency of traditional energy production technologies of comparable size. Fuel cell systems are widely appreciated as a key future energy production technology.
In order to maximize the performance and lifetime of fuel cell systems accurate control of the operating conditions for fuel cells is specified. Fuel cells produce DC current. In higher power systems, AC output is desired and thus a power conversion from DC to AC is specified. To allow for practical interfacing and current collection from the fuel cells and subsequent power conversion, the fuel cells are manufactured as stacks containing several series connected individual cells.
In fuel cell systems having several stacks, the electrical interconnection topology of the stacks is a key design parameter. Series connection of several stacks provides for lower cabling and power conversion losses as well as lower cost for components. However, a setback is that all stacks in the series will have the same current. Ideally, when all stacks are identical and their operating conditions are fully equal, this is not an issue. However, in a practical system there will always be some variations in temperature, fuel flow and characteristics of the stacks. In particular in systems designed to allow for replacement of individual stacks there may be significant variations between stacks due to age differences. When dissimilar stacks are placed electrically in series, their current can be limited according to the worst performing stack in the series. Thus the fuel cell system loses the potential performance of the healthier stacks.
Electrical parallel connection of stacks is problematic particularly in high temperature fuel cell systems due to the intrinsic negative temperature coefficient of their internal resistance. This characteristic gives rise to issues with uneven current sharing even when strings of several series connected stacks are connected in parallel. To avoid the current sharing issues, separate converters for each stack are often used and this brings additional cost to the system.
Finnish patent publication FI118553 B1, discloses a biocatalytic fuel cell arrangement where fuel cells are connected in parallel or in series. This arrangement includes controllable switches that are controlled by using a control circuit so that the switches change cyclically to and from conducting state as an object to increase the output voltage of the biocatalytic fuel cell arrangement. FI118553 B1 does not present a solution to the described problem of dissimilar stacks placed electrically at least in serial connection where their current has to be limited according to the worst performing stack in the series. In addition, FI118553 B1 does not include an embodiment where stacks can be individually measured and adjusted to avoid the dissimilarity problems between the stacks in serial connection.